In man, normal urinary bladder contractions are mediated, (inter alia), through cholinergic muscarinic receptor stimulation. Muscarinic receptors not only mediate normal bladder contractions, but may also mediate the main part of the contractions in the overactive bladder resulting in symptoms such as urinary frequency, urgency and urge urinary incontinence.
After administration of fesoterodine and other phenolic monoesters of formula (I) to mammals, such as humans, these compounds are cleaved to form the active metabolite. The active metabolite is known to be a potent and competitive muscarinic receptor antagonist (WO 94/11337). Therefore, fesoterodine and other phenolic esters of formula (I) represent potential prodrugs for the active metabolite, and are drugs which are effective in the treatment of overactive bladder with symptoms of urge urinary incontinence, urgency, and urinary frequency, as well as detrusor hyperactivity (as described e.g. in U.S. Pat. No. 6,713,464).
A synthesis for the production of the active metabolite as well as its phenolic monoesters such as fesoterodine has previously been described, e.g. in U.S. Pat. No. 6,713,464.
According to U.S. Pat. No. 6,713,464, the phenolic monoesters of the active metabolite are prepared as follows:

A solution of 2-(3-diisopropylamino-1-phenylpropyl)-4-hydroxymethylphenol (the active metabolite) and the corresponding acid chloride in dichloromethane is cooled to 0° C. Subsequently, a solution of triethylamine in dichloromethane is added dropwise during 5-10 minutes under stirring. Stirring is continued for 18 h at room temperature, and then the mixture is washed with water, aqueous sodium hydrogen carbonate, and water. The organic phase is dried over sodium sulphate and evaporated under reduced pressure. The oily residues obtained are finally exposed to high vacuum for 2-4 h, to remove the remaining traces of solvents.
The synthesis of the active metabolite, which is used as the starting material in the present invention, is known in the prior art. WO 94/11337 and WO 98/43942 both describe a multi-stage process to synthesize the active metabolite.
In accordance with general acylation procedures, triethylamine acts as an acid scavenger in the prior art process, thereby drawing the equilibrium of the reaction to the side of the end products, and increasing the yield of the phenolic monoester of the active metabolite. However, contrary to the skilled person's expectations, the applicant realized that the chemoselectivity of the reaction is superior when the reaction is performed in the absence of triethylamine.
As a consequence, there was a desire for a base which acts as a catalyst for the regioselective acylation and in comparison to triethylamine results in a higher yield and purity of the phenolic monoesters of formula (I).
Surprisingly, this object could be attained by performing the reaction in the presence of N,N-diisopropylethylamine (Huenig's base).